Abstract: System for determining relative positions and/or relative motions between objects (103, 602, 604). Disclosed systems include a target tracking device (110, 620, 622) on a first object (602) and an optical target on a second object (604). The target tracking device comprises a camera to take images of the optical targets which is configured to track the optical target. The system comprises an onshore crane (601), a ship system (700), an aviation system (800) or an offshore crane (100). A controller (115, 615, 715, 815) controls the crane, a ship (701) or a helicopter (801) using the data from the tracking device.

Abstract: Disclosed is a riser tower structure including a plurality of elongate objects. The riser tower structure is provided with blocks, preferably of buoyant material, along at least part of its length. The blocks provide the riser tower with a substantially circular cross-sectional profile, wherein one or more strakes are provided on the outside of the blocks. The strakes in a main embodiment are helical. Also disclosed is a corresponding method of constructing such a riser tower structure.

Abstract: Disclosed is a riser tower structure including a plurality of elongate objects. The riser tower structure is provided with blocks preferably of buoyant material, along at least part of its length. The blocks provide the riser tower with a substantially circular cross-sectional profile, wherein one or more strakes are provided on the outside of the blocks. the strakes in a main embodiment are helical. Also disclosed is a corresponding method of constructing such a riser tower structure.